#include <fstream>
#include <cmath>
#include <iomanip>

using namespace std ;

ifstream f ("aria.in") ;
ofstream g ("aria.out") ;

struct P {
    double x , y ;
};

P v[100005] ;
int n ;
double arie ;

int main ()
{
 f >> n ;
 for ( int i = 0 ; i < n ; ++i )
    f >> v[i].x >> v[i].y ;
 v[n] = v[0] ; //usurarea calcului
 for ( int i = 0 ; i < n ; ++i )
    arie += ( v[i].x * v[i+1].y - v[i].y * v[i+1].x ) ;
 g << setprecision(5) << fixed << abs ( arie / 2.0 ) ;
}